Cellulose esters (CEs) are conventionally synthesized by the reaction of cellulose with the anhydride or anhydrides corresponding to the desired ester group or groups, using the corresponding carboxylic acid as diluent and product solvent.
In these processes, the reaction mixture is heterogeneous initially due to the insolubility of cellulose in most organic solvents including carboxylic acids. The reaction is terminated when the cellulose derivative has gone into solution and the desired solution viscosity has been reached. When the mixture becomes homogeneous, the cellulose is fully or almost fully acylated.
Optionally, one may use a large excess of sulfuric acid catalyst, in which case the product is a cellulose alkanoate sulfate. Selective cleavage of the sulfate groups can afford a partially substituted cellulose alkanoate. It is, however, extremely difficult to remove a large DS (degree of substitution) of sulfate esters without simultaneously reducing the DP (degree of polymerization) of the cellulose ester to unacceptable levels.
Thus, in conventional processes, the synthesis of partially substituted cellulose esters is accomplished by hydrolysis of cellulose triesters, prepared by mineral acid catalyzed acylation in a separate step, to the desired level of substitution. Typically, hydrolysis in a mixture of water and carboxylic acid solvent results in scrambling of position of substitution (due to acyl migration and simultaneous, but slower, reesterification of the newly exposed hydroxyl groups by the carboxylic acid solvent) so that the products have an equilibrium distribution of ester substituents.
Partially substituted cellulose esters have great commercial value. They are used in coatings, where their greater solubility (in comparison with triesters) and hydroxyl group content (to facilitate crosslinking) are prized. In plastics, fibers, and film applications, the ability to synthesize partially substituted CEs permits control over thermal, mechanical, biodegradation, and compatibility properties.
It is well known in the art that esters of cellulose with long-chain carboxylic acids could be prepared by acylation with the corresponding acid chlorides in pyridine or, less successfully, other solvents. This method was useful only for synthesis of cellulose triesters. For example, see Malm, et al., Ind. Eng. Chem., 1951, 43, 684-688.
U.S. Pat. No. 2,705,710 discloses DMAC as a solvent and sulfuric acid as a catalyst to make cellulose triacetate (a fully substituted ester--2.9 DS Ac and 0.10 DS sulfate). The reaction disclosed in this patent is run at 140.degree. C. and, therefore, is very fast. The disadvantage of the sulfuric acid technology of U.S. Pat. No. 2,705,710 is the need for a hydrolysis step in order to obtain partially substituted cellulose esters.
Direct synthesis of partially substituted CEs has also been taught previously by acylation of cellulose in solution as shown in U.S. Pat. No. 2,976,277. If cellulose is first dissolved in a mixture of lithium chloride and an amide solvent (either 1-methyl-2-pyrrolidinone (NMP) or N,N-dimethylacetamide (DMAC)), it can then be acylated with a carboxylic anhydride in the presence or absence of a catalyst to afford a partially or fully substituted CE depending only on the equivalents of anhydride added. Esters of cellulose with long-chain carboxylic acids have been made in this way. Thus, in Carbohydrate Polymers, 22, 1-7, 1993, it is disclosed that it is possible to react cellulose in DMAC/LiCl solution with a variety of carboxylic acid chlorides using amine catalysis, or alternatively carboxylic acids using dicyclohexylcarbodiimide catalysis, to obtain esters of cellulose with acids of chain length up to 18 carbons (stearate) and DS 0.1 to 2.5. While this method has great flexibility in terms of the nature of the anhydride and the DS of the product obtained, the necessity for dissolving cellulose means that reaction mixtures must be dilute (no more than 5% cellulose) and that the process is lengthened by the time it takes for cellulose dissolution. It is a practical necessity to develop a method to recycle the expensive lithium chloride with high efficiency, which method has not yet been disclosed.
Long-chain (carbon chain length greater than 4) esters of cellulose (LCCEs) are known from the pioneering work of Malm as shown in Ind. Eng. Chem., 43, 684-691, 1951. Efforts to obtain LCCEs by reaction of cellulose with long-chain anhydrides in carboxylic acid solvent with mineral acid catalysis have not been successful because the esterification rate is too slow and cannot compete with the rate of chain cleavage.
The only other methods known in the literature involve the use of "impeller" reagents such as chloroacetic anhydride, as disclosed in U.S. Pat. No. 1,880,808, and the reaction of regenerated cellulose with long-chain acid chlorides in pyridine or, as disclosed in Ind. Eng. Chem. Res., 31, 2647-2651, 1991, neat. The impeller reagents tend to be expensive, toxic, and difficult to handle.
Regenerated cellulose is expensive, as are acid chlorides, which also require reactors of corrosion-resistant construction. Additionally, direct reaction of cellulose with acid chlorides under vacuum does not result in homogeneous, soluble products.
LCCEs are of interest commercially because of their lower processing temperatures, greater impact strength, greater solubility in less polar solvents, the likelihood of greater compatibility with hydrophobic polymers, the potential for formation into molded or extruded objects without the need for a plasticizer, and their potential utility as associative thickeners for water-based paints (by analogy with long-chain ethers of cellulose, such as hydrophobically-modified hydroxyethylcellulose).
It has been also disclosed in U.S. Pat. No. 2,705,710 that activation of cellulose with N,N-dialkylamides prior to conventional (mineral acid and carboxylic anhydride) esterification permits rapid esterification without excessive degradation. The patent also discloses that this is a process for making cellulose triacetate with inherent viscosity in the range of 1.1 to 1.3 (less than that required for many current commercial applications).
In U.S. Pat. No. 2,861,069, it is disclosed that sulfonated polystyrene resins are effective catalysts for acylation of cellulose by carboxylic anhydrides in the corresponding carboxylic acid solvent. Disadvantages of the method include: (1) that a large excess of anhydride is required to prevent undesirable chain degradation, and (2) that only fully substituted esters may be prepared in this way unless a separate hydrolysis step is included.
Clearly, a need exists in the art for a process by which CEs of less than full substitution can be prepared directly from cellulose. The process must be economical, practical, and amenable to industrial production. It should be possible with this process to synthesize products which have sufficiently high molecular weight for their particular commercial application. It would be desirable to be able to use long-chain anhydrides in this process, such that cellulose esters containing long-chain ester groups could be obtained. It is essential that the products be sufficiently homogeneous so that they can be processed thermally and/or in solution, to be useful for film, coatings, plastics, and certain other applications. It is desirable to use a catalyst which could be physically separated from the product by simple techniques such as filtration and which could, therefore, easily be recovered for re-use. It is also desirable to have the ability to control product DS and molecular weight by practical and predictable adjustments to process conditions.